This invention relates generally to missiles adapted to be launched from a launch tube and more particularly to an erodable spin turbine mounted on the periphery of the missile nozzle to impart spin to the missile during the launch phase.
Atmospheric free-flight missiles are spin stabilized to enhance their targeting accuracy by reducing ballistic dispersion resulting from nozzle defects and asymmetries in the thrust-producing exhaust gases; spin stabilization is also required for missiles with internal guidance systems. The missile velocity at the exit of the launch tube is generally insufficient to achieve adequate spin stabilization due to the airflow over the aerodynamic foils mounted on the missile's airframe; this results in an initial perturbation of the trajectory until aerodynamic spin stabilization is achieved. Therefore it has been necessary to devise means for imparting angular momentum to the missile during the launch phase so that as the missile exists the launch tube it is spin stabilized at the free-flight spin rate and the spin rate thereafter is maintained aerodynamically.
The prior art is replete with means for imparting an angular torque to a missile during its launch phase so that it is spin stabilized as it exits the launch tube. One solution to providing spin stabilization during the launch phase is to provide a launch tube with interior helical guides or grooves that the missile's aerodynamic foils ride against, so that as the missile traverses the launch tube a spin is imparted to the missile. The spin rate imparted to the missile is a function of launch velocity and helix angle. Exotic manufacturing techniques are required to produce launch tubes with helical guides or grooves which increases the per-unit cost of the missiles. This limitation is amplified when it is noted that in many applications of tube-launched missiles the launch tube is used only once. In addition, this device requires a machined clamp ring bore lay to prevent missile balloting as it traverses the launch tube; this also increases the production expense.
Canted fins or helical flutes may be inserted in the nozzle to deflect a portion of the exhaust gas stream thus imparting an angular torque to the missile. The major limitation of these devices is the loss of axial thrust due to the portion of the exhaust gas stream which impinges on these deflection surfaces. Even if the deflection surfaces are designed to burn out early in the missile's flight trajectory, asymmetrical thrust may be generated due to nonuniformity in burn out of the deflection surfaces. The cost and difficulty of production is increased for nozzles having internal deflection surfaces.
Another solution is a consumable insert for the nozzle so that the exhaust gas stream may initially be directed tangentially to the axis of the missile to impart spin to the missile. As the insert is consumed a greater portion of the exhaust gas stream is ejected axially until the insert is totally consumed at which time all of the exhaust gas stream is directed axially. A major limitation of such a device, however, is the loss of axial thrust during the launch phase, and the possibility of asymmetrical thrust generation due to nonsymmetric consumption of the insert. Since the insert must be manufactured with precision to ensure symmetrical thrust the overall cost of the missile will increase. A retaining means must also be produced to hold the insert in the nozzle which adds to the per-unit cost of the missiles. Additional time and effort is required to integrate the consumable insert and retaining means within the missile nozzle.
Deflection surfaces, attached to the outer surface of the nozzle and held in a retracted position by the launch tube so that the exhaust gas stream impinges upon the retracted deflection surfaces, may be used to impart an angular torque to a missile during the launch phase. In one alternative, as the missile exits the launch tube the centrifugal force generated by the spinning missile causes the deflection surfaces and their attachment means to be ejected from the missile. This separation can be hazardous to equipment or personnel in the vicinity of the launcher; the separation may also cause a perturbation in the trajectory of the missile. In the other alternative the deflection surfaces are withdrawn from the exhaust gas stream by a bias means as the missile exits the launch tube, but remain attached to the missile. A disadvantage of these devices is that they have a negative effect on the aerodynamic characteristics of the missile. At a minimum the coefficient of drag of the missile will be increased; the deflection surfaces may also cause asymmetric aerodynamic forces to act on the airfoils to increase the ballistic dispersion of the missiles. Either type of deflection device increases the cost and complexity of producing the missile, and as the number of mechanical elements is increased the reliability of the missile decreases.
Finally a separate gas stream may be directed to canted surfaces on the airframe of the missile to impart an angular torque to the missile in the launch tube. This method requires auxiliary equipment to generate the gas stream which increases the cost of the system; also the launch tube must be modified to function with this system which will increase the cost of the system. If the canted surfaces remain on the airframe during the free-flight trajectory negative aerodynamic forces will act on the missile. If the canted surfaces are consumable there is a likelihood that the airframe of the missile will also be scored as the canted surfaces are consumed.